Combined amplitude and frequency modulation detectors



Jul 17, 1951 E. I. ANDERSON COMBINED AMPLITUDE AND FREQUENCY MODULATIONDETECTORS 3 Sh'eets-Shegt 1.

Filed May 17, 1946 INVENTOR 5424 j flA/osaram BY 4 MWL/ 'ATTORNEY July17, 1951 E. I. ANDERSON 2,561 ,088 COMBINED AMPLITUDE AND FREQUENCYMODULATION DETECTORS Filed May 17, 1946 3 Sheets-Sheet 2 INVENTOR 44;1'. Amoaarom ATTORNEY July 17, 1951 I E. l. ANDERSON 5 L COMBINEDAMPLITUDE AND FREQUENCY MODULATION DETECTORS Filed Nay, 1'7, 1946 i 3Sheets-Sheet 5 'Tm' .4 75A 75-- 7a AKA [M 485 70 INVENTOR BY /wvm,

ATTORNEY Patented July 17, 1951 COMBINED AMPLITUDE AND FREQUENCYMODULATION DETECTORS Earl I. Anderson, Manhasset, N. Y., assignor toRadio Corporation of America, a. corporation of Delaware Application May17, 1946, Serial No. 670,386

3 Claims. (Cl. 250-27) My present invention relates to detectors offrequency modulated (FM) or amplitude (AM) carrier waves, and moreparticularly to novel combined AM-FM detectors.

In his application Serial No. 614,956, filed September '7, 1945, now U.S. Patent No. 2,497,841, issued February 14, 1950, Stuart W. Seeley hasdisclosed and claimed a novel detector of angle modulated carrier waveswhich is insensitive to amplitude variations of the waves. The detectorproduces output only in the presence of a variation in the ratio ofsignals, such as frequency modulation (FM) signals, applied to a pair ofrectifiers having but a single direct current path connecting them inseries-aiding polarity. Such a ratio detector produces modulation signaloutput in response to variation in the ratio of the FM signal strengthat the respective rectifiers, while the common direct current pathreferred to above carries substantially all the unidirectional currentwhich flows through either rectifier. Various proposals have been madein the past for selectively detecting FM or AMsignals in a commondemodulating system. However, such prior combined FM-AM detector systemsare not applicable to the Seeley FM ratio detector circuit.

It is, therefore, an important object of my present invention to providea combined FM-AM receiver employing a ratio detector of the typedisclosed in the aforesaid Seeley application; at least one of therectifiers of the ratio detector being adapted for detection of AMsignals.

I It is a further object of my invention to pro- Videan FM detector ofthe aforesaid Seeley type, wherein existing circuit components of thedetector are readily utilized to provide detection of AM carrier wavesof a different frequency than Waves in the FM range.

Still other objects of my invention are generthe detector circuit in,Fig. 1

Fig. 3 is a circuit diagram of a modified embodiment of the FM-AMdetector circuit;

Fig. 4 is afurther modification; and

Fig. 5 shows the equivalent circuit diagram of the AM detector circuitin Figs. 3 and 4.

Referring now to the accompanying drawings, wherein like referencecharacters in the different figures designate similar circuit elements,Fig. 1 shows an illustrative receiving system embodying a demodulatornetwork adapted to provide audio voltage and automatic volume control(AVC) voltage in response either to FM or AM signal reception. Thereceiver circuits prior to the demodulator network are schematicallyrepresented. Those skilled in the art of radio reception are wellacquainted with the nature of the circuits customarily employed inmulti-band receivers. While my invention is readily adapted for FM andAM signal reception on respective bands of 88 to 108 megacycles (mc.)and 550 to 1700 kilocycles (kc), it is to be clearly understood that theinvention is not limited to such frequency bands. The 88 to 108 me. bandis given by way of illustration, since it is the FM broadcast band nowassigned to such transmission. The invention is applicable to the 42-50mo. band, so far as FM reception is concerned. The 550 to 1700 kc. bandis the present AM broadcast band assigned to transmission of'AM signals.

It will further be understood that in the following description andclaims the generic expression angle modulated is intended to includefrequency modulation, phase modulation or hybrid modulations thereofhaving characteristics common to both FM and phase modulation. From avery general viewpoint my invention relates to a demodulator networkhaving separate input circuits for carrier waves of differentfrequencies and of different modulation characteristics.

, The numerals l and 2 in Fig. 1 denote respectively different sourcesof modulated carrier waves. Source I may be the usual signal wavecollector, such as a dipole, employed for collecting FM signal Waves.The FM signal waves are transmitted from FM transmitters at a mean orcarrier frequenc assigned to the particular transmitter. In the assumedFM band of 88 to 108 mc. the radiated carrier wave frequency would beselected from that range, and would be transmitted as a wave of variablefrequency and substantially uniform amplitude. As is well known, thefrequency modulation of the carrier wave would be in accordance with themodulation 0 (such as audio) frequency signals at the transmitter. Theextent of frequency deviation of the carrier frequency is a function ofthe modulation signal amplitude, while the rate of frequency deviationis dependent upon the modulation signal frequencies per se. The presentpermissible extreme frequency deviation in the FM band of 88 to 108'mc.is '75 kc.'to eitlier side'of fix the carrier frequency, while theallotted channels are 200 kc. wide. These values are pure lyillustrative. Source 2 may be the customary groundedantenna circuitemployed in AM broadcast reception. The allotted AM broad'dast" channelsare. 10'

kc. wide in this band. In AM'broadcas't trans: mission the carrier waveis modulated in amplitude in accordance with the modulation; or audio,

frequency signals. The carrier frequency is maincomponents of thedemodulator network cooperatewith coil 8 to provide selective detectionof either. the :FM signals or the AM signals, and, in

addition, provide automatic volume control '(AVC) voltage during thedetection of either 'formof received signals.

The demodulator network itself comprises but ztwo electron dischargedevices, shown as diodes tained constant in value at the transmitter.The

numeral 3 designates a tunable radio frequency amplifier havingsuitablesignal selectorcircuits for FM orAM reception.-

Switching 'devices'd and 5 respectively provide separate connection ofthesources IandZ t respective selector'ci'rcuitsof amplifiers" It willbe understood that when switch 4'*is in': closed position, collected FMsignal energy will be'applied to the selector-circuits of amplifiers.Such selector-circuits are not shown, but it will be understood thatthey are capable ofselectivelyamplifying "the FM signals over a band atleast 150kc': wide. Upon closing of switch 5 and-opening switch 4', thesame amplifier 3 will have the FM- selector circuits operativelyreplaced by AM selector circuits; These latter circuits willselectthecollected- AMsignals andpermit amplifier 3 to amplify the sameover a 10 kc: band. Multiband-selector circuits and-switchin devices-forsuitable change-over are well known to those; It is to be understoodthat theswitching devices 4 skilledin the art of radi communication.

and- 5 affect the detector network only in-so far as they determinethecharacter of the modulated wave which is delivered to the demodulatorinput circuit.

' Assuming" that the system is of the superheterodyne type, as is theusual practice-at-present, the converterfii and intermediatefrequency(-I. F.) amplifier "I will also' be'provided with suitable FM and AMsignal selector circuits. Atthe amplifier I, which may consist of oneor'more .separate stages of amplification, will have an ultimate outputcircuit across which is'developed" the amplified FM-signals orAM'signals atthe respective I. F. values. the variable inductance type,isto' be understood as beingarranged in series in the plate" circuit ofthe last I. F. amplifier tube.

of a suitable direct current voltage source: The" numeral Qdesignates aniron core, or slug, which is utilized to vary," or'adjust, theinductance "of" coil 8 to a suitable desired value. The'condenser' I 0'bypasses 'the lower 'end' of coil '8 to ground for I. F. currents.

There are to be'developed across the'coil fl'th'e FMsi'gnals at the10.7'mc; mean frequency'when' The coil 8; which is of Hence; the lowerend of coil 8 is shown connected to apoint'H-B' by yvay of illustration.The electrodes of the pair "ofliio'ds may be housed within a common tubeenvelope, or they may be located in separate envelopes-I- TheFM-detector 'circuit' is substantially that disclosed and "claimed in theafore' said Seel'ey application: Accordingly, referenceis made to theSeeley application for-a" detailed explanation of the constructionandfunctioning of an FM'ratio detector circuit'of the type shown in Fig. 1.It will be observed that the present-re-'- ceivin'g system, as explainedin'the Seel'ey'appli-- cation; dispenses with an'amplitudelimiter priortothe input section (or FM'tran'slating network) of the detector.

One of the reasons in-tliepast'for employingan amplitude limiter priorto the in section of" the FM demodulator to' reduce undesired effects onthe carrier wave, was't'o avoid" the necessity for critical tuningtotheexact' center," or 'carrier, frequency of a desired FM"w'ave. As

explained in the aforesaid Seel'eyapplication,

there n'eed be no special amplitudelimiter-stage employed prior to thedetector circuit, since the detector itself is substantially immune toampli tude variations of the received FM signals; Hence, the I. F.amplifier immediatelypreceding the-detector circuit may possessnormaland full gain; which is thereverse' of theusua'loperatingcond-ition for an amplitude limiter.

The input network of the FM detector coin-- prises coupl'ed primary and"secondary circuits denoted-by numerals lIand l2 respectively: The inputcoil 8"isindicated as'apart "of the'primary circuit. Whileany known andsuitable discr'iin inator circuit'may be uti-lize'd'to providethe-energizing signal'voltages for diod'e-rectifi'ers l 3" and' anglemodulated waves "a'pai'r'of voltages whose" relative amplitudes vary inaccordancewith the angular deviations of the waves with respectfto" apredetermined reference condition (whether phase or frequency)Considering the specific" illustrative embodi ment of "'Fig'. 1, andassuming-that the-receiver is"*adjusted for FM si'gn'al reception, coil'Isis.

shunted by condenser I6 to provideaparallel resonant circuit tuned tothe operating ILF. value of '10.? inc; Condenserlli need'not'be'aphysical capacitor as such, but may be the sum of'th'e' capacitancesappearing across coils flfand l5? The secondary coil l1 is coupled to;primary: coil I5 as indicatedbynuiiieral IBfah'd coll IT is shunted bycondenser I9. The resonant secondary circuit I2, including coil I1 andcondenser I9, is tuned to substantially the resonant frequency of theprimary circuit Each of coils I5 and Il may be of the known inductancetrimmer type, or capacity tuning may be used. Specifically, iron coresor slugs may be used for adjusting the inductance values of therespective coils I5 and Il, if coil I! is so arranged that varying theslug does not unbalance the two halves of the coil. The high alternatingpotential side of coil I5 is connected by a condenser 20 to the highpotential side of coil 8. Lead 2| connecting the midpoint ll of coil I1to the intermediate tap I5 of coil I5 establishes the mid point ll andintermediate tap I5 at a common potential. Tapping down on coil I5 provides improved impedance matching and greater sensitivity for the ratiodetector. In addition, the proper ratio of primary to secondary voltagemay be obtained which improved the linearity.

Rectifiers I3 and I4 may have their electrodes embodied in a common tubeenvelope, as in the 6H6 type tube. The cathode 29' of diode I3 isconnected to the upper terminal, as diagrammatically shown, of condenserI9 and to the upper end of coil I1, whereas the anode 26 of diode 4 isconnected to the lower terminal of condenser I3 and to the lower end ofcoil II. The anode 22 of diode I3 and the cathode 23 of diode I4 aredirectly connected together through condenser 24 when the metallicarmature 24' is in the FM position for making the electrical connectionto the contact point 24". The cathode 23 and the corresponding terminalof condenser 24 are established at ground potential for direct current.The magnitude of condenser 24 is chosen so that the anode 22 of diode I3is at ground potential with respect to modulation frequencies i. e.,audio frequency as well asfor I. F. Grounding this point 3| provides anegative voltage at the anode 22 of diode |3 which is utilized forautomatic volume control.

The anode 22 of diode I3 is, in addition, connected to grounded cathode23 by a pair of seriesarranged condensers 25 and 26. Each of thesecondensers 25 and 26 has a relatively low impedance to I. F. currents,and they function as I. F. bypass condensers. The primary coil I5 hasits right hand end or terminal connected to the junction of condensers25 and 26. Hence, the right hand terminal of coil I5 is at groundpotential for I. F. currents, since condenser 26 connects that point toground. The coil 8 functions as a radio frequency choke, since it feeds+B voltage to the I. F. amplifier plate. Since condensers 20 and 26 areof low impedance, their reactance is effectively in shunt with thereactance of inductance I5 and capacitor I6.

It can be seen that the diodes I3 and I4 are arranged in reversedrelation relative to the connection in a conventional FM detectorcircuit of the type employing balanced diodes. The detector circuit iscompleted by a resistor 21 which is shunted by the condenser 24. It isagain emphasized that the switch armature 24 is in electrical connectionwith the contacts 24" and 21', the so-called FM position.

The modulation voltage, in this case the desired audio frequency signalvoltage, is taken off by connecting lead 28 to the low I. F. potentialend of primary coil I5 1. e., the junction of condensers 25 and 26.Condenser 29 is an audio frequenc coupling condenser, and is inserted inthe lead 28, which includes the resistor 28, to

the, input grid of the following audio amplifier tube (not shown). Thejunction of condenser 23 and resistor 28' is connected to ground throughresistor 30, and the second FM switch armature 3| is-assumed to beconnected to the contact 32 (which is' connected to the ungrounded endof resistor 30) and to the contact 33 which is connected to groundthrough condenser 34. Resistor 28' and condenser 34 provide ade-emphasis time constant to compensate the pre-emphasis applied at thetransmitter.

The AVC path from the detector circuit includes the connection whoseinput end is connected to the'contact 27' by means of the tappedresistor 4| and lead 42. The right hand end of resistor 4| is connectedto ground through the condenser 43, while the left hand end of resistor4| is connected to contact 44. The AVC lead 40 is connected through theusual filter resistors to the respective controlled signal grids ofthe'amplifier 3, converter 6 and I. F. amplifler I. Those skilled in the artof radio communication are fully acquainted with the specific detailsfor connecting the AVG circuit into thevarious networks whose gains areto be controlled in response to the AVG voltage derived from thedetector circuit.

The switch armature 24, which is permanently connected by lead 46 to theanode 22, is adjusted into AM receiving position by disconnecting thearmature 24 entirely from the contacts "24" and 21. I have shown the AMposition in dotted lines. The switch armature 3| is adjusted to AMposition when it is shifted into electrical connection with contacts 32and 44. Here, again, the dotted line representation on contacts 32 and44 designates this AM position of armature 3 I. It will be noted that inthe AM position of armature 3| the contact 33 remains free, while theAVG lead 40 is connected in circuit with diode I4. On the other hand,when armature 24' is shifted to its AM position, the diode I3 is out ofelectrical circuit with the output connections of the detector network.

In order to explain the operation of the receiving system shown in Fig.1, let it first be assumed that the armatures 24' and 3'! are in theirFM positions as shown in Fig. 1. In this connection of the detectornetwork, it being assumed that the preceding circuits of the receivingsystem have all been conditioned to receive FM signals, the detectorcircuit functions substantially as describedby S. W. Seeley in hisaforesaid application Serial No. 614,956. It is not believed necessaryto explain the operation of the FM detector circuit operation in detail,since such operation has been described with considerable detail andexplanation in the aforesaid Seeley application.

It is believed suflicient for the purposes of the present application topoint out that due to the coupling I8 between tuned circuits II and I2there will be a phase shift between the primary and secondary voltageswhen the instantaneous carrier frequency (10.7 m0.) is at the resonantor center frequency value. Accordingly, the secondary signal voltage atcircuit I2 will be applied to cathode 26 and anode 26 from therespective ends of coil I1 in opposite phase, but in each case in phasequadrature with the primary signal voltage. or course, it is assumedthat the FM signals across coil 8 are instantaneously at the mean orcarrier frequency of 10.7 me. It follows, therefore, that the resultantsignal voltages applied to cathode 23 and anode 20 will be intenseequai' ait 'th'e carrier frequency, and'tneireeufi'd voltageswiubeofequaima 'nitude. 1 Iff'at' some laterJin'sta'nt' the nMsignais have afrequency difi erent from the resonant frequency of circuit f2," therewill occura phase shift of; the signal energ .transmi'tted thi'ough the.trans former 15," IT whichis greater or less tiianeoi' depending uponthe direction .and 'the extenti'of' frequency difference between theinstantaneous frequency of the FM signals. andithe' sweeter minedresonant frequency o'f the tunee circuits H" and. 12'." This means" thattherewiirbeiappiied. to the diodes l3 and It resultant signal-voltages;of different magnitudes. Therefore,Ttlierectified' voltages will"b'e ofdifi'e'rentlmagnitudes;

As. pointedv outfini the aforesaid :seele'y tappll cation, ifior somereason the magnitude fof'ape' plied I.'F. signals suddemyicropswizard-tee. FM detector circuit 'tendst'o maintain. the output potentialthat. was assumed-.Qbefore th' signal was removed, and" it does this?rather than return to the Zero or average potential" as it"does in the.conventional and know liiniters-di's'c'ri i?- nator' combination. Itwill" be' noted that the. detector cir'cuit' has but-a singledirectlc'urrent. path connecting the diode rectifiersfin series-aid? ingpolarity. The condenser 25 shunted across resistor 2T acts to inhibitchanges in thelvolta'ge drop across resistor 2'7 at a modulation'frequency rate. The underlying or' basic -n'iode or ratio detection maybe stated-in these siiriple' I terms: The direct current'potentialk'across two diodes l'fii'and i l in series, each passing lthesamedirectv current, will be directlyflproporti'tinalfl the I."F. energyapplied'..to those two diodessub-'- stantially regardless of themagnitudeofftl'iat direct current.

The audio frequency signals, correspondingto the. modulation applied tothe FMv carrier at. the. transmitte-r,.. are taken off from. thejunction of. condenser 25. and 26' through-lead 2till-resistor 28?. andcondenser 29. The .A'VC connectionismade to the. anode 22' throughthepath .includingree sistor '4 l',.'1'ead' '42 contact.2i',- armature24'I'and leadill'fil. Ithas been pointed-outintheaforee said Seel ey.application that groundingthe.anode 2 2'. ofldi'b'de .13 formodulationrfrequencies.i. egaudio frequency. aswell as I. .;F., provides"a negative voltage at-the anode 22which-may:be used forautomaticflgaincontrol.

As. explained heretofore, the cost of a;.com-'- bined A M-FM receiverwill be substantiallyire-l; duced if one. or both of the diode-e131,: ik-used;

in the-.FM-ratio detector circuit'can, a1so,-beused-- as. therectifier.for AM- detectiom As explained heretofore, the .FM detectorcircuit is-very, simply), converted: to. an AM detector circuit by-merely.

shifting the switch armatures 24 and.3'|"'t0Z th:- dottedline positionsAM.

In accordance with my invention; the-coiL-a is chosen in magnitude so asto resonate at the LF. value in AM reception, which is 456-kc., withthecapacities of condensers 20 and 26 inseriesw full effect. ofresistor 4iand condenser li-forthis purpose.

Assuming-now that thearnr'iatures 24" an'di3f have. beenshift'e'd totheir respective AM'po'si'- tions, a'ndthe preceding circuits of thereceiving systemhave been-conditioned for AM signal'r eception,.-the.resulting AM detector circuit as depictedin Fig.2, which been redrawn asthe equivalent detector circuit. In other words; inf ig. 2l't'here havebeen eliminated thos'e circuit'coniponents which are effectively cutofcircuit-la't' therelatively low I. F. frequency used-in AM r-ece'ption.These components are principally the inductances IT'and lS- andtheirrespective tuning capacitors. I'Q- and N3 The dioderecti fier I4is-the. actual AM signal-rectifier, and diodeand condenser 25' playnopart intLloeAli/fdetection'. However, the diode l- 3 and condenser 25cannot be disconnected without-switching hot? radio frequency. circuits.However, these two elements will-not afiect the operationof. theresultingA-M detector circuit. Accordingly,.it will be--notedthattheeffective AM detector circuit consistsoithcceil 8 shunted-by condensers20 and 2:6- in series, the diode M,- having its anode 20- connected tothejunction of condensers '20 and:-

2B;- whi1e the cathode-23 oi diode I4 is grounded;

The diode I4 is shunted by resistors 28" and 3D- series,-.condenser 29transmitting the audio frequency voltage from the-junction ofresistors-28" and 30. TheAVC lead 40 is now connected throughvresistor M to thejunction of resistors 28' and 30,- while the: filter condenser 43 con'nects the-right'hand' end of resistor 41- to ground.- Itwillbenoted-that the network 4|, 43 serves toremove the audio frequencycomponentexisting" acrossresistor 30-when the direct current-voltagecomponent is employed-for AVCbias during AM reception: The-voltageappliedto diode rectifier- I4 is less thanthe voltage-appearingacross-coil 8-.-- The actual fraction-of the voltage-appearing acrosscoil 8 which'isappliedto-rectifier M for AM detection- 1 is determinedbythe ratio of Y the reactance of=condenser 2-6 to the sumof there--actancesoi condensers'20 and 25. The-reactance ofi=condenser-20 may'bem'ade small compared-tothe reactance 'of condenser 26, andinthis'case the voltage. 1oss:is--sm-all.-- It should be noted that duringdetection condensers 2!! and 26 are connected in series across coil 8,and-efiectively tune -coil8 totheoperating I. F. value: of 456 During FMsignal-reception the condenser 25 is efiectively in parallel withcondenser 26,. thereby changing the apparentcapacityacross coil-8 atthe-I ...F. value of 456--kc. (AM- reception) This detunesthe detectioncircuit when receiving: in-the FMbandsothat their is'less likelihood-0fFM.-signa1-energy coming through at theAM-irequencyandcausingdifficulty.

It willnow be appreciated that I have utilized the existing circuitcomponents of theSeeley FM ratio. detector circuit for- AM signaldetection;

By adjustingea pair of:- switches in the-detectorcircuitzcertai-n of thecircuit components are effec-- tively removed irom the circuit, whilethe coil- 8' coacts with-the existing condensers wand-26 and-di0derectifier Hi to provide a simpleiAM. detector-circuit. Aspoin'ted outpreviously lead- 2| from mid-point IT" on coili'i connectsto anintermediatetap on'coil I5, rather than to. the

high -potentialside of. primary circuit-H This is done: orderto improvethe gain of the' dete'c' A cult of thediodes l 3 andi'l 4".

In the modification of my invention shown in Fig. 3 I have shown adifferent form of. coupling circuit between the plate of the driver I.F. ampli fiers tube (not shown) and the primary circuit H. In place ofthe coil 8 of Fig. 1, there is em played a parallel resonant circuit 50consisting of coil and shunt condenser 52. The circuit5ll is value at FMreception. The condenser is a small capacitor whose value is equal to K/o,-.o16 The capacitor 20 can be replaced by mutual inductance couplingbetween coils 5| and I5. The use of tuned impedance inthe plate circuitof the driver tube (I/F. amplifier) provides more efficient coupling andhigher gain.

To provide AM detection upon adjustment of suitable switches, I insertcircuit 53 in series with circuit 50. The circuit 53 consists of coil 60and parallel condenser 6| tuned to the operating I. F. value (456 kc.)for AM reception. In series with primary circuit N there is connected aresonant circuit 62 tuned to 456 kc. Circuit 62 consists of coil 63 inseries with coil I5, while condenser 64 shunts coil 63 to tune it to the456 kc. frequency. If the magnitudes of condensers are so chosen thatthen condenser 20 will provide the proper coupling in both the AM and FMbands. If mutual coupling is provided between coils l5 and 5|, thenseparate coupling must be provided between coils 60 and 63.

The remainder of the changes necssary in the circuit of Fig. 3 follow.Th series-connected condensers and 26 are shunted by series-relatedresistors 65 and 66, whose junction is connected to a contact 61 (AMcontact). The switch arm S1 is connected to the junction of condensers25 and 26, while contact 68 (FM contact) is connected to ground by ade-emphasis condenser 69. The audio voltage, whether appearing as AM orFM on the received carrier, is taken off from junction point 25' by thecondenser 10. Resistors 65 and 66 are roughly the same as resistor 30 inFig. 1.

Switch S2 is provided by a pair of spaced contact arms 1| and 12.Contact arm H is connected to the anode end of resistor 65, whilecontact arm 12 is connected to the same point through the AVG filterresistor 13. The FM contact of arm returns to ground through resistor 21shunted by condenser 24. The'FM contact of arm 12 is connected by lead12 to resistor 65. The AVC line includes the alternating voltage filterresistor 13' and condenser 74, and is to be connected, as shown in Fig.1, to the controlled amplifier stages. In Fig.3 the switches S1 and S2are shown adjusted for FM signal reception. Resistances 65 and 66 willpreferably be high compared to resistance 21.

The operation for FM reception is exactly the same as in Fig. 1. It willbe observed that for AVC voltage the ratio detector acts as a voltagedoubler, while this is not true for audio voltage. When the switches S1and S2 are adjustedfor AM signal reception, the switch S1 will beshifted to contact 61, and switch S2 has its arms H and I2 shifted tothe respective AM contacts. In effect this removes circuits 50, H and i2from the input to diodes l3 and I4, since the reactancesof thesecircuits are low at the 456 kc. frequency of AM reception. Because ofthe fact that resistance 27 is much smaller than resistances 65 and 56in series, the gain of the I. F. circuit for AM reception is greatlyreduced when receiving FM signals.

The equivalent circuit diagram in Fig. 5 shows the AM detector circuitresulting from adjusting switches S1 and S2 in Fig. 3 to the respectiveAIM positions. As shown, the diodes 3 and 14 act as a voltage doublerAVC rectifier network, while for audio voltage the output of onrectifier is used. The input circuit efiectively consists of circuits 53and 52 coupled in cascade by condenser 20. Of course, if the capacity 20Were replaced by mutual inductance, then the latter would be thecoupling reactance. Since resistor 2'! and condenser 24 are out of thecircuit-on AM, they are not shown in Fig. 5.

If the audio voltage in Fig. 3 were taken from the upper end of resistor65 so that the output of-both diodes l3 and 4 were used, the circuitwould be a voltage doubler for audio voltage in AM signal reception.Good design would provide the same audio output for a given settin ofthe volume control on both AM and FM reception. Hence, on AM receptionit may be necessary to take even less than the audio from one tube.

The AVC voltage may be too great, also, in either or both the AM and FMsignal bands, and so the AVC may, also, be derived from a tap on theresistors 65, 66.

Thecircuit of Fig. 3 may be modified to the 1 cuit 2 are replaced by theparallel resonant circuits 10. and 1|. Thus, circuits 10 and H arerespectively arranged in series with respective diodes l3 and M. Theresonant circuit 62, comprising coil 63 and shunt condenser 64, isarranged in series between the junction of circuits 10 and II and thejunction 25' of condensers 25 and 26. Coil 5| is magnetically coupled toeach of circuits l0 and H, and the latter are respectively tuned to 10.8mo. and 10.6 me. In other words, the driver circuit 50 is coupled tooppositely mistuned resonant circuits 1!], II in the manner shown inConrad, U. S. Patent No. 2,057,640 of October 13, 1936. The AM circuits53 and 62 are magnetically coupled at coils 60 and 63. For FM signalreception the Conrad discriminator input circuit functions to providethe amplitude-variable signal voltages for diodes l3 and M. For AMsignal reception the circuit effectively resolves itself into theequivalent AM detector circuit of Fig. 5.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular circuitorganizations shown and de scribed, but that many modifications may bemade without departin from the scope 0t my invention.

What I claim is:

1. In combination with a source of angle modulated carrier waves and asource of amplitude modulated carrier waves, an inductance elementtively deriving direct current-voltage either fromsaid resistoror fromsaid output circuitand' for selectively disconnecting said resistor fromsaid" rectifirs; said fi-r'st and third'capacitorshavingsucl'ia-capacitance-=as to tune said inductance e1e-'- ment-tonthe frequency of 'said' amplitude modu lated-carrier waves.=.

23 The combination-with a source of angle mod ulated carrier waves and asource of amplitude modulated carrier 'waves, of an: inductance ele mentcoupled to said source; a' frequency discrim inator network responsive'to said angle modulated carrier waves: arfirst capacitor coupling said'inductance -element to saidnetwork, a-resistor,a

painofrrectifiers' connected to said networkand series 1 withz:saidresistor to 1 provide a direct current'path' therethrough; a secondand a third capacitorrh'avingra low impedance to said anglemodulatedcarrier waves and connected across said resistorpthe junctionpoint r of said 1 second: andethird' capacitor being connected throughsaid? network to said firstcapacitor; amodulation ire:-

quency :output; circuit: coupled to. said 2 junction point, v a circuitincluding; switch a means forr selectively deriving direct currentvoltage eitherz f rormsaidresistorzor if romssaid output circuit, 'andfurther iSWitChZIIlBEIlST'fOI' selectively disconnects ing: said:resistor from said 'rectifie'rs;.said first: and thirdncapacitorsihaving such; a capacitance; as to" tune; said:ginductancez-element tothe frequency: of said;amplitude modulated :carrier :waves;

3.,iIn combination witha source ,of anglewomode ula'te'd-carrier- "wavesand a source of amplitude modulated carrier' waves, afirst inductanceele-- ment coupled-to said sources, a" frequency dis.- cri-minator"network comprising a first parallel resonant circuitr including a secondinductance element, a second parallel resonant circuit including athird}, inductance element,,, said .resonantcircuitsbeing -tuned-to thefrequency of said angle modulated carrier waves," a first o capacitorcouplingsaid'ifirst'element to said first resonantlcircuityatconductiveconnection between interme-): diate pointsjof saidsecond and'saidthird element,

aresistor; a-pair -of rectifiers' connected to said second--resonant"circuit and J in series with ,said resistor-to provide;a direct currentpath therethrough: a second andia third capacitor having a-1owimpedance-to said anglemodulated carrier waves and connected: acrosssaid resistor, the

junction point of said 'secondand third capacitor being connected ,tosaid first resonant circuit, a v modulation frequencyoutput-circuitcoupled .to i said network: and", a' circuit; including switch h meansfor selectivelyideriving. direct'current volt age-"either'from said"resistorior from .said output circuit and for selectivelydisconnecting-said re sistor'irom'said rectifiers, sa-idfirst and thirdca;-

pacitors having such a" capacitance as to tune said-first element tothefrequency of said-ampli tudemodulatedcarrier waves.

EARL I. ANDERSOLL REFERENCES/z CITED Theefollowing references are ofrecord in the filesofrfthisk-ipatentz';

UNITED STATES PATENTS Number Name- Date 2,258,599 Carlson Oct. 14, 19412382;015- Lange" Aug; 14, 1945'; 2,4135913 Duke- Janh'i, 19472j4-13;9:'77' Koch- ,Jan: '7, ,1947' 21429;762 Koch Oct. 28, 19477;

